The surface glycoproteins of enveloped viruses such as human and simian immunodeficiency viruses (HIV and SIV) mediate cell attachment and membrane fusion, thus allowing the virus to enter a new host cell. This proposal grows directly out of our recent determination, by x-ray crystallography at 3.9 A resolution, of the structure of unliganded (i.e., not bound to CD4 or to an Fab) and fully glycosylated SIV gp!20 core, the receptor-binding portion of the SIV envelope protein. Previous structural analyses of HIV-1 gp!20 core and of HIV-1 gp41 (the fusion-promoting fragment of the envelope protein) have yielded models for the postfusion conformations of these proteins, but the new structure now allows us to assess directly the conformational changes induced by receptor (CD4) and co-receptor (CXCR4 or CCR5) binding. We propose to use insights from the new structure to pursue 3 specific aims. First, we will generate stabilized forms of unliganded SIV gp120, by introduction of one or more additional disulfide bonds, to produce betterordered crystals and to provide a template for parallel design of stabilized HIV-1 gp120. Second, we will introduce similar disulfide links into HIV-1 gp120 and make other modifications, in order to crystallize it as an unliganded species. If successful, we will determine the structure in complex with 2 inhibitors that block an early step in viral entry: the small molecule, BMS-378806, and the peptide, 12pl. Third, we will design and produce stable, rigidified, and potentially crystallizable forms of the HIV and HIV gp140 trimer, the ectodomain of the form found on the virion surface. We will attempt to crystallize these trimers for x-ray structure determination, as well as to study them by single-particle cryo-EM. Our long-range goal is a complete "molecular movie" of the conformational changes in HIV/SIV envelope protein that allow it to mediate viral entry. By achieving this goal, we anticipate being able to generate concepts for novel antiretro viral therapeutics and for novel immunogens.